An arrangement in which a motor of an electric power steering system is driven via an H-bridge circuit equipped with four switching elements is known from Japanese Patent Application Laid-open No. 2000-190861. By turning ON two switching elements in a first diagonal relationship among the four switching elements of the H-bridge circuit, the motor can be driven in a forward direction, and by turning ON two switching elements in a second diagonal relationship, the motor can be driven in a reverse direction.
There is a known method for detecting an ON failure of a switching element (failure in which the switching element is stuck in an ON state and cannot be turned OFF) by monitoring the actual current that is supplied from a battery to a motor via an H-bridge circuit during operation of an electric power steering system. That is, when two switching elements in a diagonal relationship are turned ON in order to drive the motor, if another switching element has an ON failure, the high voltage side and the low voltage side of the H-bridge circuit are short-circuited via two switching elements among these three switching elements to generate an excessive current flow which would not normally flow. When current detection means detects this excessive current, it can be determined that there is an ON failure of the switching element.
However, when the voltage of the battery is low or the electrical resistance is increased due to deterioration of a harness, the excessive current becomes small, and thus there is no obvious difference from the current flowing through the motor when the electric power steering system is operating normally. By merely monitoring the current by the current detection means to check whether the current becomes an upper limit value or more, it is difficult to reliably determine whether there is an ON failure of a switching element.